Carbon Stock in Slash and Burn and Alternative Land Uses at ASB Benchmark Sites in the Humid Tropics

Entry ID:
CGIAR_ASB_CStock

Summary

Abstract:
Carbon stocks were measured in the soils and vegetation in 94 sites in the three ASB benchmark countries (Brazil, Cameroon, Indonesia) and in an additional 22 sites in Perú. The sites sampled in each country included undisturbed or selectively logged forests as the reference point; areas that had been recently slashed, burned, and cropped; and areas that were subsequently planted to pastures, ... tree plantations or agroforests, or areas abandoned to fallow regrowth. This dataset, compiled by the ASB benchmark team, is unique in that it provides data collected and analyzed by standardized methods across sites. In addition, the information in this dataset on the carbon stocks and carbon accumulation rates in young fallow vegetation and agroforestry and plantation systems are rare for the tropics.

Geographic Coverage

Spatial coordinates

N: 12.0

S: -55.0

E: 55.0

W: 10.0

Data Set Citation

Dataset Originator/Creator:
Consultative Group on International Agricultural Research (CGIAR)

Dataset Title:
Carbon stock in slash-and-burn and alternative land uses at ASB Benchmark sites in the humid tropics

Quality
The current dataset allows for general comparison of C stocks and time-averaged C values among general land-use types, but some caution must be taken in using these estimates. There are several steps in which errors may affect the accuracy of the estimates. These include small plot sizes for estimating the biomass of large ... trees, insufficient numbers of replicates, and inappropriate allometric equations for estimating tree biomass for some of the systems.

The total area sampled for tree biomass at each site was 500 m2 (= quadrat size (00m2) multiplied by five (quadrats per site). Although this may be sufficient in areas where trees are small, < 25 cm diameter at breast height (dbh), it is much less than the 2,500 m2 recommended by Brown et al., (1995) for obtaining accurate measurements in tropical forests where much larger trees are encountered. The protocol has now been modified to increase the quadrat size to 5m X 100m in areas where there are trees with a dbh >25cm.

The above- and below-ground C estimates for most of the land-use systems were obtained from only three or four true field site replicates (in each field site, estimates were obtained from an average of five quadrats = pseudoreplicates). In some cases the variability was quite low, but in others it was unacceptably large, and in other cases the estimates were obtained from only two field site replicates. If these C values are to be used for modeling and national inventories, then the accuracy must be improved by increasing the number of replicates.

Another source of error could be related to the allometric equation used for estimating the biomass of trees based on their diameter. The current equation was developed primarily for mature forests that often included only trees greater than 10 or even 25 cm in diameter (Brown et al., 1989). In addition, the density of the wood in these mature systems may be greater than that in young, regrowing systems. There are indications that this equation may overestimate the C of trees of dbh < 25cm, which, in fact, includes most of the trees in the secondary forests, fallows, agroforestry and tree plantations measured at the ASB sites. New equations being developed based on extensive sampling of trees in young fallows (Ketterings and van Noordwijk for Indonesia and Palm and Szott for Perú) give estimates half those obtained from the Brown equation. Several other recent studies have shown a considerable range in allometric equations for both primary and secondary forests in the humid tropics of Brazil (Alves et al., 1997; Araujo et al., 1999; Nelson et al., 1999).

Application of these new equations to young, regrowing fallow and agroforestry systems will affect carbon stock estimates and rates of C accumulation. Such systems are currently of interest to the global change community as there is debate on how much C is taken up by regrowing vegetation. Once new equations for smaller diameter trees and for specific agroforestry species have been agreed upon, then C stocks, C accumulation rates, and time-averaged C values for many of these systems can be improved relatively rapidly. In addition, since most of the C in these systems is in the trees, we would recommend sampling several more young fallows, mature or growing plantations and agroforestry systems. The tree biomass will be estimated by measuring dbh of the individual trees, noting which species, and then applying the specific allometric equations.

Root sampling and estimation of the C stored in roots has proven to be the most difficult of all the parameters measured. The estimates for roots have not been included in the tables and figures presented in this report. If one assumes that the root-to-shoot ratio remains relatively constant for the different systems within a site, then there is a means of estimating the C stored in the root systems. At the very least, it is possible to say that including roots in the C stock comparisons made above will only magnify the loss of C. As an example, the roots in a plantation will be less than the roots of a forest system, as is the above-ground C, and therefore the difference in total C between the two systems is larger than, but in proportion to, that estimated by above-ground C only.

Access Constraints
ASB encourages free dissemination of its work when reproduction and use are for non-commercial purposes, provided all sources are acknowledged. ASB strives for open, public access to its datasets and will consider requests for access to ASB datasets on a case-by-case basis in consultation with the ASB Consortium partners who generated the data.